Eleven invasive and metastatic tumors have been produced by implanting human tumor specimens of diverse histopathology into the cheek pouch of unconditioned, adult golden hamsters. We are currently testing the hypothesis that these malignant neoplasms resulted from an in vivo fusion of human tumor with hamster host cells, involving severe chromosome segregation and the preferential loss of human chromosomes. Recent evidence obtained by individual chromosome G-Banding supports this thesis and will be extended to several in vivo and in vitro human/hamster and murine/hamster tumor cell systems in which we believe cell fusion was implicated. The possible roles of tumor- or host-borne viruses in these events (i.e., transformation or hybridization) will be explored.